(1) Field of the Invention
The present invention relates to a method and apparatus for feedback control of the air-fuel ratio in an internal combustion engine.
(2) Description of the Related Art
As a measure taken against exhaust gas pollution and fuel consumption, a lean burn system has recently been developed. According to this lean burn system, a lean mixture sensor is provided for generating an analog current in proportion to the lean air-fuel mixture in an exhaust pipe of an engine. Thus, feedback control of the air-fuel ratio of the engine can be accomplished by using the analog output of the lean mixture sensor, thereby attaining an arbitrary air-fuel ratio on the lean side.
In such a lean burn system, when the engine is in a warming-up mode, feedback control of the air-fuel ratio for the stoichimetric air-fuel ratio has also been carried out and the exhaust gas cleaned by a three-way catalytic converter. This, naturally, reduces the fuel consumption efficiency during the warming-up mode.
In order to improve the fuel consumption efficiency during the warming-up mode, a lean burn system can be forcibly applied to a warming-up mode engine in the same way as to the after-warming-up mode engine. In this case, however, when the temperature of the engine is too low, vaporization of fuel within chambers of the engine is poor, so that the combustion of fuel is insufficient, inviting misfires and thus reducing drivability.